1. Field of the Invention
This invention relates to an image forming apparatus utilizing an electrophotographic process and more particularly to such an image forming apparatus as a copying machine, a laser beam printer, or a system of facsimile which forms an image by effecting primary transfer of a toner image formed on an image bearing member provisionally onto an intermediate transfer member and secondary transfer of the toner image so received on the intermediate transfer member onto a transfer material by means of a contact transfer member.
2. Related Background Art
The image forming apparatus which attains the formation of an image by effecting primary transfer of a toner image formed on a drum-shaped electrophotographic photosensitive member as an image bearing member (hereinafter referred to as a "photosensitive member") provisionally onto an intermediate transfer member and secondary transfer of the toner image so received on the intermediate transfer member onto a transfer material by means of a contact transfer member serves effectively as a color image forming apparatus or a multicolor image forming apparatus which produces an image by subjecting the plurality of component color images of a color image information or a multicolor image information to sequential superposing transfer. Thus, a color image or a multicolor image or as an image forming apparatus which is endowed with the ability to form a color image or the ability to form a multicolor image is synthetically reproduced. It can obtain an image which does not incur any misregister of the component color images (misregister of colors).
In the image forming apparatus constructed as described above, it is important for the purpose of obtaining a perfect image to ensure removal of the toner which remains on the intermediate transfer member after the secondary transfer of the toner image from the intermediate transfer member to such a transfer material as sheet. For the conventional image forming apparatus of this class, the following methods (a)-(d), for example, have been proposed for the removal of the toner which remains on the intermediate transfer member.
(a) A method which comprises causing an elastic blade to make and break contact with the intermediate transfer member thereby scraping the residual toner on the intermediate transfer member (as disclosed in JPA 56-153357 and JPA 5-303310, for example).
(b) A method which comprises providing the intermediate transfer member with a far brush adapted to make and break contact therewith, applying to the residual toner on the intermediate transfer member a bias of the reversed polarity relative to the residual toner and consequently recovering the residual toner, causing the recovered toner provisionally to adhere to a bias roller such as a metallic roller, and scraping the toner from the bias roller with a blade.
(c) A method which comprises causing the residual toner on the intermediate transfer member to return to a photosensitive member by means of an electric field with a view to allaying the burden of the blade cleaner mentioned above (as disclosed in JPA 4-340564 and JPA 5-297739, for example).
(d) A method which comprises providing an charging device adapted to charge the residual toner on the intermediate transfer member to a reversed polarity relative to the charged potential of the photosensitive member, and causing the residual toner on the intermediate transfer member to return to the photosensitive member solely by the operation of the charging device for precluding the wastefulness of providing similar cleaning devices severally for the intermediate transfer member and the photosensitive member and simplifying the construction of the cleaning device (as disclosed in JPA 1-105980, for example).
Incidentally, the methods of (a) and (b) mentioned above which clean the intermediate transfer member of the residual toner solely by the mechanical force of the elastic blade or the brush have the problem of incomplete cleaning due to readily leakage of the residual toner through the blade or the brush. They also have the problem of further incurring incomplete cleaning due to gradual wear of the cleaning member resulting from a repeated cleaning operation.
The cleaning method of (c) mentioned above which utilizes in combination a mechanical force and an electrostatic force is effective in respect that it differs from such mechanical means as mentioned above. This method nevertheless has the problem of necessitating a cleaning step for removing the residual toner on the intermediate transfer member separately of the standard print step, suffering the inability to allow continuous printing of images of different patterns, and lowering notably the throughput of the image formation.
The method of (d) mentioned above seems to be an effective means because the construction which comprises providing an charging device adapted to charge the residual toner on the intermediate transfer member to a reversed polarity relative to the charged potential of the photosensitive member, and causing the residual toner on the intermediate transfer member to return to the photosensitive member solely by means of the charging device is very simple. Similarly to the method of (c), however, this method necessitates a cleaning step for removing the residual toner on the intermediate transfer member separately of the standard print step which implements the formation of an image.
Again in this case, therefore, the decline of throughput of the formation of image poses a problem.
Then, a failure to charge uniformly the residual toner on the intermediate transfer member prevents the residual toner on the intermediate transfer member from being electrostatically returned to the photosensitive member and results in incomplete cleaning, i.e. persistence of part of the residual toner on the intermediate transfer member.
Further, in the apparatus which, for improving the throughput of the formation of image, is constructed such that a next toner image on the photosensitive member is transferred onto the intermediate transfer member at the same time that the residual toner on the intermediate transfer member is returned electrostatically to the photosensitive member, the incomplete cleaning mentioned above constitutes itself a serious problem because it affects the next image.